The Earth is inclined at an angle of 23.5 degrees with the vertical.  So the rays of the sun are slanted at different angles.  When the rays are more direct, it is warm while when the rays are slanted it is cold.  Around March 21, the Sun is positioned directly over the Equator.  There are exactly 12 hours of daylight and 12 hours of night all over Earth.  This day is called the Vernal Equinox.  Vernal is Spring, and Equinox means equal division.  This is the beginning of Spring in the Northern Hemisphere.  During Spring, the position of the Sun appears to move Northwards, (days in the northern hemisphere last longer) until it reaches 23.5 degrees North  The line on Earth corresponding to 23.5 N is called the Tropic of Cancer.  (Tropic means the turning point). The date is around the 21st of June, and there is maximum amount of daylight.  The Sun momentarily seems to stand still, and so this is called the Summer Solstices.  It is the first day of Summer.  Days now start getting shorter in Summer, as the Sun seems to move South.  Around September 21, the Sun is again over the equator, but now it is the beginning of fall and is called the Autumnal Equinox.  Days and nights are again exactly 12 hours.  In Fall the days in the northern hemisphere continue to become shorter.  Around December 21, the Sun reaches its Southernmost limit, and the day is known as the Winter Solstices (standing still).  The line on Earth is called the Tropic of Capricorn.  Then in Winter, the days in the Northern Hemisphere lengthen until on the next March 21, the Sun is back over the Equator, and the cycle is repeated.  In the Southern hemisphere, the seasons are reversed with Fall commencing on March 21.

Summers are warmer because in Summer, the rays of the Sun strike the Earth at an angle closer to the vertical, while in winter the rays are slanted and strike at an angle closer to the horizontal.    The Earth's orbit around the Sun is elliptical, with the Sun being at one of the foci.  The Sun is at the closest point to Earth on January 4 (Perihelion) (Peri = close, helion = Sun), and furthest on July 5 (Aphelion).  So in the northern hemisphere, the Sun is closer to the Earth in Winter and further away in Winter.  But the difference between Perihelion and aphelion is very small (the orbit is almost a circle) and so this has very little effect on the temperature. For a more concise explanation Seasons
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 What does The Milankovitch Theory say about future climate change?  Orbital changes occur over thousands of years, and the climate system may also take thousands of years to respond to orbital forcing. Theory suggests that the primary driver of ice ages is the total summer radiation received in northern latitude zones where major ice sheets have formed in the past, near 65 degrees north. Past ice ages correlate well to 65N summer insolation (Imbrie 1982). Astronomical calculations show that 65N summer insolation should increase gradually over the next 25,000 years, and that no 65N summer insolation declines sufficient to cause an ice age are expected in the next 100,000 years (Hollan 2000).

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